1. Field of the Invention
The present invention relates to a shift control valve of a hydraulic control system for automatic transmission, and more particularly to the shift valve which can provide a hydraulic control system for automatic transmission having a simple and compact construction by concentrically arranging a manual valve in a valve body, and ensures safety by maintaining a formal shifting operation when an electronic control system is out of order.
2. Description of Related Art
A conventional automatic transmission for a vehicle has a torque converter and a multiple stage transmission gear mechanism connected with the torque converter, which includes a hydraulically-actuated friction member for selecting one of a plurality of gear stages of the transmission gear mechanism in accordance with vehicle operating conditions.
A hydraulic control system pressurized by a fluid pump provides a working pressure required to operate the friction members and control valves.
The commonly-used automatic transmission for a vehicle has the torque converter which generally includes a pump impeller connected with an engine output shaft to be driven thereby, a turbine runner with an output shaft member, and a stator disposed between the pump impeller and the turbine runner, so that hydraulic fluid is circulated by the engine-driven pump impeller through the turbine runner with the aid of the stator which functions to deflect the hydraulic fluid from the turbine runner to a direction where fluid flow does not disturb a rotation of the pump impeller when fluid flows into the pump impeller.
An automatic shift is made by an operation of the friction members such as clutches or a kick-down brake at each shift change. Also, a manual valve, a port of which is converted by selecting a position of a selector lever, is designed to be supplied with a fluid pump and to supply the fluid to a shift control valve. In a 4-speed automatic transmission, the shift control valve has an opening port changed by an electronic control system.
FIG. 6 is a side sectional view for depicting a connecting state of a conventional shift control valve and manual valve, where two shift control solenoid valves 1, 2, operated according to a signal of a transmission control unit TCU, are connected with a manual valve 3 via line 4, and a line pressure is regulated in accordance with an on/off operation of the shift control solenoid valves 1 and 2.
At this point, the regulated line pressure is designed to work on ports 5a,5a',5b of the shift control valve 5 to supply a hydraulic pressure to plugs 6,7 which is limited in its movement by stoppers 8,9.
A valve spool 10 is disposed between the stoppers 8,9 such that the hydraulic pressure supplied from an inlet port 11 formed on a valve body is selectively supplied to a first outlet port 12, a second outlet port 13 and a third outlet port 14.
To supply the hydraulic pressure selectively, the valve spool 10 includes a first land 15 and a second land 16 smaller than the first land 15. The first and second lands are limited in its movement by a first stopper 8 and a second stopper 9, respectively.
Further, the ports 5a,5a' communicate with each other and are designed to supply the hydraulic pressure to the first and second plugs 6,7, respectively. The port 5b independently supplies the hydraulic pressure to the valve spool 10, and the plug 7 is elastically supported by a spring S thereby receiving a constant force which is to move toward the second stopper 9.
The manual valve 3 includes an inlet port 18 receiving the hydraulic pressure from a fluid pump 17, an outlet port 19 supplying the hydraulic pressure from the inlet port 18 to the inlet port 11 of the shift control valve 5 and a valve spool 25 which moves in accordance with a movement of a shift lever.
The valve spool 25 includes a land 21 for supplying the hydraulic pressure to the outlet port 19 when the shift lever is shifted to one position of "D" position, "2" position and "L" position and a land 23 for supplying the hydraulic pressure to a port 26 when the shift lever is shifted to an "R" position.
In the conventional hydraulic control system as described above, in a first speed stage of "D" range, since both of the shift control solenoid valves 1,2 are turned ON by the TCU, the hydraulic pressure generated from the fluid pump 17 flows into the inlet port 18 of the manual valve 3 and then flows into the inlet port 11 of the shift control valve 5 via the outlet port 19. However, since the hydraulic pressure is exhausted through an exhaust port Ex, the valve spool 10 moves leftward by the hydraulic pressure working on the first land 15.
That is, as shown FIG. 6 depicting a first speed stage of the "D" range, in the first speed stage of "D" range, a friction element is operated by the hydraulic pressure supplied through a line L diverged from the outlet port 11 of the manual valve 3.
Further, in a second speed stage of the "D" range, since the shift control solenoid valve 1 is turned OFF by the TCU, the hydraulic pressure does not work on the port 5b of the shift control valve 5 but works on the ports 5a,5b.
Accordingly, the first and second plugs 6,7 move to and stop by the first and second stoppers 8 and 9.
At this point, the first plug 6 pushes the valve spool 10 which penetrates the first stopper 8, such that the second land 16 of the valve spool 10 moves to a right side of the first outlet port 12 whereby the first outlet port 12 communicates with the inlet port 11.
Accordingly, the hydraulic pressure generated from the fluid pump works on another friction element through the first outlet port 12 thereby realizing the second speed stage.
Further, in a third speed stage of the "D" range, both the solenoid valves 1,2 are turned OFF by the TCU so that the first and second outlet ports are opened. In a fourth speed stage of "D" range, the shift control solenoid valve 2 is turned OFF so that the first, second and third outlet ports are opened to supply the hydraulic pressure to each friction element thereby realizing the speed change.
However, in the above-described hydraulic pressure control system, since the manual valve and the shift control valve is separately disposed from each other, a problem that a space for equipping the valves needs occurs. Besides, since two shift control solenoid valves are used, there occurs another problem that control is complicated.
Additionally, when the electronic control system is out of order, since the two shift control solenoid valves are turned OFF, the third speed stage of "D" range is maintained, such that an engine brake effect can not be obtained thereby having a problem in safety.